In optical networks, optical components such as optical add/drop multiplexers are used to add and drop channels from optical communication signals such as Dense Wavelength Division Multiplexed (“DWDM”) optical signals. For example, a DWDM signal may have 160 channels, and those 160 channels (numbered n=1 to 160) each carry data at a certain optical carrier wavelength (λn). In such an optical network, the add-drop multiplexers can pull single or multiple communications channels off from the incoming multiplexed is optical signal and can selectively add new communications channels into the outgoing multiplexed optical signal.
Known add/drop multiplexer systems use fiber Bragg gratings as principal elements. These known systems are reconfigurable through the use of tunable optical filtering elements. For example, the Bragg gratings can be temperature-tuned, such that the reflection wavelength is shifted in or out of an optical channel band. Operationally, such Bragg gratings can be tuned to lie between optical channel bands or tuned to an optical channel band. When the grating reflection wavelength is tuned to an optical channel band, the corresponding optical channel is dropped. When the grating reflection wavelength is tuned out of an optical channel band, the corresponding optical channel is a “through” channel, i.e., a channel that is transmitted unaffected through add-drop multiplexer. In another technique, the gratings are mechanically strained to shift the reflection wavelength in or out of optical channel bands. Alternatively, the gratings can be thermally or mechanically tuned to different optical channels. In this manner, control of either temperature or mechanical strain functions to create a reconfigurable add-drop multiplexer. Exemplary such systems are described in U.S. Pat. Nos. 5,712,932, 5,748,349, 5,778,118, 5,786,914, 6,108,468, and 6,256,428.
A plurality of these gratings can be lined up along a fiber in order to pull off multiple channels, but each such grating inevitably causes some signal insertion loss and/or interference and/or cross-talk between channels. Thus, reconfigurable add-drop multiplexers that can add and drop multiple channels are difficult to implement efficiently using known technologies.